1. Field of the Invention
The present invention relates to a phase-change type optical recording medium, particularly to an optical recording medium capable of repeatedly recording information at a high recording density.
The present application claims priority from Japanese Application No. 2000-394353, the disclosure of which is incorporated herein by reference for all purposes.
2. Description of the Related Prior Art
There has been known a phase-change type optical recording medium which is capable of recording, reproducing and erasing information by virtue of a laser beam irradiation. A basic principle for recording, reproducing and erasing information in a phase-change type optical recording medium can be explained as follows. Namely, a laser beam is used to irradiate a recording layer formed on a substrate to effect a reversible change between an amorphous state and a crystal state on information recording areas of the recording layer. In more detail, when information is to be recorded in an optical recording medium, a laser beam having a high output and a short pulse is used to irradiate the recording medium to heat information recording areas to a temperature equal to or higher than their melting point, followed by a quick cooling of the heated areas, thereby forming recording marks in amorphous state. On the other hand, when the recording marks are to be erased, a laser beam having a lower output and a longer pulse than those used during the recording is used to irradiate the recording layer, in a manner such that the recording marks are heated to a temperature equal to or higher than their crystallizing temperature, followed by a gradual cooling treatment, thereby rendering the recording area to return back to the crystal state and thus erasing the recording marks. Further, when information recorded in the optical recording medium is to be reproduced, a laser beam having a lower output is used to irradiate the information recording layer, so as to read the recorded information by virtue of change in the reflectance of the recording area.
Furthermore, the above-described phase-change optical recording medium is capable of over-write recording by modulating the intensity of a single laser beam. During the over-write recording, the above-described recording and erasing are performed simultaneously such that when new information is recorded, recorded information is at the same time erased, thereby making it possible to omit an independent erasing process. At this time, the single laser beam is modulated between a recording power (capable of heating the recording layer to a temperature equal to or higher than its melting point) and an erasing power (capable of heating the recording power to a temperature equal to or higher than its crystallizing temperature and lower than its melting point), in a manner such that the laser beam is modulated to the recording power in areas where recording marks are to be newly formed, but modulated to the erasing power in other areas. In this way, irrespective of whether the recording layer before irradiation is in an amorphous state or a crystalline state, the areas irradiated by a laser beam having the recording power will all become into amorphous state, while the areas irradiated by a laser beam having the erasing power will all become into crystalline state, thereby effecting the over-write recording.
Since the above-described phase-change type optical recording medium is capable of performing recording, reproducing and over-writing by modulating the intensity of a single laser beam, such an optical recording medium is considered to be an excellent recording medium since it is allowed to use a simplified optical system on its drive side and also since the recording medium is Interchangeable with other existing read-only disc such as CD-ROM, rendering itself extremely useful as a recording media having a large storing capacity and capable of performing a rewriting process.
A conventional phase-change type optical recording medium comprises a first dielectric layer, a recording layer, a second dielectric layer, a reflecting layer and a protection layer, successively laminated on a substrate. The substrate is formed by a material substantially transparent with respect to a laser beam (for recording and reproducing information) passing from below. In fact, such a material may be a glass or a resin such as a polycarbonate. The recording layer is formed by a material in which phase-change will occur upon being irradiated by a laser beam. In practice, such a material forming the recording layer may be a chalcogenide material such as Gexe2x80x94Texe2x80x94Sb.
The first dielectric layer, the second dielectric layer and the reflecting layer are provided to prevent oxidization of the recording layer (so as to prevent a deterioration possibly caused due to repeated over-write recording), and to adjust a heat response during recording process, as well as to enhance an optical effect during reproducing process. With regard to an optical enhancement effect, the first dielectric layer produces a multiple interference effect between the substrate and the recording layer, while the second dielectric layer produce a multiple interference effect between the recording layer and the reflecting layer. In this way, it is possible to increase a reflectance changing amount of the recording layer, thereby improving the data quality of the recording medium. Here, the first and second dielectric layers are formed mainly by ZnS and SiO2, while the reflecting layer is formed by a metal having a high reflectance, which may be a monomeric metal such as Al, Au and Ag or an alloy. The protection layer is provided for preventing the recording medium from getting wounded and for ensuring sufficient corrosion resistance. In fact, the protection layer is usually formed by a resin.
On the other hand, since the above-described phase-change optical recording medium is required to have a higher recording density, it has been considered necessary to shorten the wavelength of the laser beam, reduce the thickness of the substrate, improve the aperture number of an objective lens, and select an appropriate recording manner (land groove recording). In order to produce such a high density recording medium, there have been at least the following approaches which are: 1) using a blue-violet semiconductor laser having a wavelength of about 400 nm; 2) employing near-field recording/reproducing technique using SIL (Solid Immersion Lens), 3) reducing land-groove width in land-groove recording.
However, the above approaches have been found to have the following problems, which are: 1) the blue-violet semiconductor laser requires the use of a light source having a high and stable output and a long lifetime, but such a light source has not yet been in its reliably useful form; 2) the near-field recording/reproducing technique fails to have an interchangeability with other recording medium since it employs a special lens; 3) reducing land-groove width requires to reduce the width of groove walls so as to ensure a necessary width for land upper surface or for the flat portion of groove bottom surface. With regard to problem 3), although it is preferable that an angle approximately equal to 90 degrees be formed between the groove wall surfaces, this will make it difficult to form a recording medium substrate by virtue of injection molding.
In order to solve the above problems so as to realize a high density recording, it has been suggested to use an objective lens having an aperture number of 0.55-0.70, and to use a laser beam having a wavelength of 600-700 nm, thereby making it possible to use an optical recording medium having a plurality of V-shaped grooves concentrically or spirally formed on the substrate. At this time, since a groove pitch is also a track pitch, a track pitch of 0.7-0.8 xcexcm, preferably 0.5 xcexcm or less is useful for realizing a high recording density.
FIG. 3 is an explanatory view showing a conventional phase-change type optical recording medium adopting the above suggestion. In the drawing, reference numeral 1 is a substrate having a plurality of V-shaped grooves 1a formed concentrically or spirally on a substrate. These grooves are arranged at a pitch p, each groove having a depth d and a half-value width w. A first dielectric layer 2, a phase-change type recording layer 3, a second dielectric layer 4, a metallic reflecting layer 5 and a protection layer 6 are successively laminated on the substrate 1. A laser beam having a wavelength of 600-700 nm travels from below the substrate 1 so as to form recording marks on the recording layer 3 in areas corresponding to the grooves 1a. In this way, by virtue of refraction caused due to inclined surfaces of grooves 1a, since a laser beam converged by an objective lens may be further converged, it is allowed to obtain an effect equivalent to that obtainable when improving the aperture number of an objective lens. Furthermore, since light components not received by the grooves 1a are not converged into the recording marks, and since these light components will not be detected out as reproduction signals, it is sure to prevent a cross talk and a cross erase, thereby ensuring a high recording density.
However, with regard to the above-described phase-change type optical recording medium including the substrate, the first dielectric layer, the phase-change type recording layer, the second dielectric layer and the metallic reflecting layer, there has been a problem that over-write recording can cause an undesired jitter at an extremely early time. In addition, according to the results of the experiment conducted by the inventors of the present invention, it is understood that if a recording medium substrate is a polycarbonate disc including a plurality of grooves having a track pitch of 0.7 xcexcm, a depth of 20 nm and a half-value width of 0.35 xcexcm, and if an over-write recording is performed for 1000 times, an actually allowable limit (jitter 9%) will be undesirably broken.
It is an object of the present invention to provide an improved optical recording medium having a high recording density and a good over-write performance, which is so formed that even if a recording medium substrate has a plurality of V-shaped grooves, it is still possible to reduce jitter and increase the number of times for over-write recording.
According to the present invention, there is provided an optical recording medium comprising: a substrate including concentrically or spirally formed grooves each having a V-shaped cross section; a first dielectric layer formed on the substrate; a phase-change recording layer formed on the first dielectric layer; a second dielectric layer formed on the phase-change recording layer; and a metallic reflecting layer formed on the second dielectric layer. In particular, the optical recording medium is so formed that an objective lens having an aperture number of 0.55-0.70 can be used and a laser beam having a wavelength of 600-700 nm and passing through the objective lens is passed through the substrate and then converged on the recording layer, thereby recording or reproducing information on the recording layer in areas corresponding to the grooves. Specifically, the optical recording medium further comprises a third dielectric layer consisting of a nitride, the third dielectric layer being disposed between the first dielectric layer and the phase-change recording layer.
In one aspect of the invention, the third dielectric layer mainly contains silicon nitride or germanium nitride, and has a thickness of 1-7 nm.
In another aspect of the invention, each of the first and second dielectric layers contains zinc sulfide and silicon oxide.
In further aspect of the invention, the phase-change recording layer mainly contains In, Ag, Sb and Te.
In one more aspect of the invention, the grooves are periodically meandered, having a track pitch of 0.7-0.8 xcexcm, a depth of 20-30 nm and a width of 0.35xc2x10.05 xcexcm.
In still one more aspect of the invention, the metallic reflection layer mainly contains Al and Ti, with Ti being 0.5-0.5 atom %.
According to the above constitution, since the third dielectric layer consisting of a nitride and having a desired softness is provided between the first dielectric layer and the phase-change recording layer, it is sure to prevent a deterioration possibly caused due to a heat between the phase-change recording layer and the first dielectric layer, thereby improving a reversible characteristic between an amorphous state and a crystal state on the phase-change recording medium. In this way, it is possible to alleviate a deterioration possibly caused by jitter and to increase the number of possible times for over-write recording, even under a condition where an optical recording medium is used which has a high recording density and includes a substrate having V-shaped grooves.